It is well known to those skilled in the art that polymers and gelled or crosslinked water-soluble polymers are useful in enhanced oil recovery and other oil field operations. They have been used to alter the permeability of underground formations in order to enhance the effectiveness of water flooding operations. Generally, polymers or polymers along with a gelling agent such as an appropriate crosslinking agent in a liquid are injected into the formation. The polymers then permeate into and gel, in the cases when a polymer and a crosslinking agent are used, in the regions having the highest water permeability.
Polymers have also been used in subterranean formation treatments such as "matrix acidizing" and "fracture acidizing". Because such treatments are well known to one skilled in the art, description of which is omitted herein and can be found in U.S. Pat. No. 4,997,582, description of which is herein incorporated by reference.
Because of environmental concerns as well as cost for disposing of a produced brine which is defined as the brine co-produced with oil and gas and is generally contaminated with some oil, or gas, or both, it is desirable to utilize the produced brine as the liquid used for the polymers and appropriate crosslinking systems. Use of produced brines eliminates not only the cost associated with acquiring and pre-treating fresh water for use as the liquid but also the disposal cost for the produced brine. Most produced brines are known to be hard brines, i.e., those having a divalent cation concentration greater than 1000 ppm.
Many polymers have been developed and used in processes for the recovery of hydrocarbons. Generally a desirable property is that such polymers impart to a liquid an increased viscosity when a relatively small quantity of the polymer is added, and preferably at a minimal cost. Another desirable property is that such polymers form gels, in the presence of a suitable gelling agent such as a crosslinking agent. However, a number of such polymers are not capable of forming gels having high thermal stability, i.e., the gels formed show high syneresis after a short period, such as for example a few days, at high temperature, such as for example, 120.degree. C. in a harsh environment such as sea water.
Various polymers may be used in the process for recovery of hydrocarbons. For example, metallic ions crosslink gellable polymers through the interaction with the oxygen atoms of the polymer molecules. Therefore, the gellable polymers generally contain some carboxylate groups. Generally, the gellable polymers used such as, for example, polyacrylamide are of high molecular weight and contain high degrees of hydrolysis, i.e., contain 10-30 mole % carboxylate groups. However, these high molecular weight and/or high mole % carboxylate group-containing polymers gel almost instantly in the presence of the above-described multivalent metallic compounds. Such fast gelation rate renders the application of gelling compositions containing these polymers and multivalent metallic compounds not useful in many oil-field applications such as, for example, water shut-offs and permeability reductions.
Many processes have been developed to delay the gelation of gelling compositions by adding a gelation delaying agent to the gelling compositions. However, a gelation delaying agent is not inexpensive and a gelation delaying agent often adds appreciable costs to oil field operation. Furthermore, many gellable polymers cannot withstand a hostile environment as described above.
There is therefore an increasing demand for water-soluble polymers that can be used to prepare gels which withstand hostile environments. A hostile environment includes, but is not limited to, high temperatures, high salinity and/or high content of multivalent metal cations, commonly known as "hardness ions", as well as the high acidity, temperature and shear conditions encountered in processes such as acid fracturing.
In the art of drilling wells to tap subterranean deposits of natural resources, such as gas, geothermal steam or oil, it is well known to use a drilling fluid. In addition to having the desirably rheological properties such as viscosity and gel strength, it is very important that such drilling fluids exhibit a low rate of filtration or water loss, that is, the drilling fluid must prevent excessive amounts of fluid, or "filtrate", from flowing from the bore hole into the surrounding formation. The loss of water or other fluid from the drilling hole is prevented by the formation of a filter cake which deposits from the drilling fluid and seals the wall of the bore hole. Numerous formulations, compositions and additives to optimize the performance of drilling fluids for various applications have been developed. For instance, compositions comprising mixtures of carboxylic acid polymers and soluble metal salts with the object of increasing the "yield" (defined as the number of barrels of 15 centipoise mud which can be prepared from one ton of clay) of relatively low-grade clays have been used.
Excessive fluid loss from the drilling fluid may contaminate the producing formation, permanently displacing oil and blocking production. The adverse consequences of excessive fluid loss in the drilling of very deep wells are more severe due to the high temperatures and pressures encountered in such drilling operations. The viscosity of a fluid normally decreases with an increase in temperature, but certain polymer additive or deflocculating agents may reduce, or even reverse, this tendency. However, the polymers which are most effective in achieving this effect are the most vulnerably to breakdown through oxidation, shear and thermal effects, i.e., the duration of exposure to high temperature drilling operations. Also, many such polymers tend to precipitate and/or lose viscosity as well as effectiveness as water loss additives when exposed to dissolved electrolytes, particularly when divalent metal cations such as Ca.sup.+2 and Mg.sup.+2 are present. In drilling fluids, the resulting vulnerability to breakdown is exacerbated by the density of drilling mud, which is directly related to weighting agents required for a given formation pressure.
Breakdown of polymers causes a large increase in the fluid loss accompanied by an increase in filter cake thickness. These conditions often result in differential sticking of the drill string. It is, therefore, desirable to develop additives which enable drilling fluids to retain their proper viscosity and fluid content over a broader range of conditions.
Drilling fluids are used in the drilling of various types of wells. Workover and completion fluids, in contrast, are those fluids used in the completion and servicing of such wells. Completion fluids are those fluids used after drilling is complete and during the steps of completion, or recompletion, of the well. Completion can include cementing the casing, perforating the casing, setting the tubing and pump, etc.
Workover fluids are those fluids used during remedial work in the well. This can include removing tubing, replacing a pump, cleaning out sand or other deposits, logging, reperforating, etc. Workover also broadly includes steps used in preparing an existing well for secondary or tertiary oil recovery such as polymer additions, micellar flooding, steam injection, etc.
Both workover and completion fluids are used in part to control well pressure, to prevent the collapse of casing from overpressure, and to prevent or reduce corrosion of casing. A drilling fluid may be suitable for completion or workover applications in some cases, but not in all cases.
Although there has been considerable progress in the field of workover and completion fluids, there is significant room for further improvement. For example, wells are being completed and serviced in increasingly hostile environments involving, e.g., high temperatures and high levels of salinity and/or hardness in the formation water. Thus, new additives for workover and completion fluids which retain their properties at elevated temperatures and high concentrations of dissolved electrolytes are in demand.
Therefore, a composition which can be used to prepare a more hostile environment-withstanding polymer as well as a hostile environment-withstanding gelling composition, containing the hostile environment-withstanding polymer, that can form stable gels in a liquid such as, for example, produced brines, for near-wellbore as well as in-depth treatments, and preferably that does not require a gelation delaying agent, is highly desirable.